The present invention relates to a circuit for preventing arcing between current carrying contact points in a relay as the contact points are opened or closed. Such arcing may occur when relay contact points are opened or closed with a potential across them. Arcing not only causes undesirable radio interference as a result of radiation broadcast from the switch, but also severely limits the useful life of the relay contact. The contact points may be charred and contact resistance increased to the point where the relay will fail to operate satisfactorily.
A number of approaches have been taken to prevent arcing across relay contact points as the points are opened and closed. U.S. Pat. No. 3,736,466, for instance, discloses a circuit in which a triac semiconductor is placed in series with the power carrying contact points of a mechanical switch. The switch includes a second set of contact points which are connected to gate the triac off during the opening and closing of the power carrying contacts and thus eliminate arcing. One drawback to such a circuit is that the triac is in series with the power source and load and thus must have a sufficiently large continuous current rating to handle the current applied to the load.
In order to use a semiconductor having a smaller continuous current rating, several circuits have placed the semiconductor in parallel with the power carrying contact points of a relay switch. U.S. Pat. Nos. 3,558,910 and 3,555,353, as well as J. W. von Brimer, "Commutated Relay Combines Solid-state Switching," April 1965, 13th Annual National Relay Conference, page 14-1, show such circuits. The circuits disclosed in the two patents both use the current supplied to the relay coil to energize the gate terminal of a triac which is connected in parallel with the current carrying contact points of the relay. The von Brimer article, on the other hand, shows a relay having a primary set of contact points and an auxiliary set of contact points. The auxiliary contact points are closed first so that current is supplied to the gate of a triac causing it to become conductive prior to the closing of the primary current carrying contact points. These three circuits have the disadvantage that the triac in parallel with the current carrying contact points is maintained on as long as the current carrying contact points are closed. If the current carrying contact points have only negligible resistance, the triac will be effectively shorted while the primary contact points are closed and will therefore carry none of the load current. If, however, the contact points should develop appreciable resistance, the triac will be forced to carry a sizable current and may therefore be overloaded.
To prevent damage to the semiconductor in such a situation, it is necessary to allow the semiconductor to conduct for only a short time interval during the closing and opening of the primary relay contact points. One approach taken to accomplish this result is shown in U.S. Pat. No. 3,639,808. The circuit there disclosed uses a D.C. supply to energize the relay coil. A secondary coil is linked to the relay coil and connected to the gate of the triac so that the gate receives a signal only when the relay is being switched on or off. This has the advantage that a smaller triac may be used since it will not continue to conduct the load current if the relay contacts should fail to close or should develop appreciable contact resistance. Such a circuit, however, requires that a D.C. supply voltage be available for energization of the relay coil.